% graphviz - FAQ
% Ivan Lazar Miljenovic
Fortuitously Anticipated Queries (FAQ)
======================================
Note that to distinguish it from [Graphviz], the library shall be
henceforth referred to as _graphviz_.
Graphviz vs _graphviz_
----------------------
### What is the difference between Graphviz and _graphviz_? ###
[Graphviz] is an open source library and collection of utility
programs using that library to visualise [graphs] (which are specified
using the [Dot] language).
_graphviz_ is a library for the purely functional programming language
[Haskell] that provides "bindings" to Graphviz's programs. It does so
by allowing programmers to specify the layout of the graph and then
converts that to Dot code before calling the appropriate program to
perform the visualisation.
[Graphviz]: http://www.graphviz.org/
[graphs]: http://en.wikipedia.org/wiki/Graph_theory
[Dot]: http://www.graphviz.org/doc/info/lang.html
[Haskell]: http://haskell.org/
### Why should I use graphviz over one of the other Haskell Graphviz libraries? ###
Various Haskell libraries have support for Graphviz to one extent or
another; however _graphviz_ has the most comprehensive support
available out of all of them:
* Two different methods of specifying Dot graphs:
1. Strict, which matches the layout of `dot -Tcanon`.
2. Liberal, which allows statements to be in any order.
There are also conversion functions between the two of them.
* The ability to parse and generate most aspects of Dot [syntax] and
[attributes]. This includes taking into account escaping and
quoting rules where applicable.
[syntax]: http://graphviz.org/doc/info/lang.html
[attributes]: http://graphviz.org/doc/info/attrs.html
* The ability to use a custom node type for Dot graphs.
* Support for the all five layout algorithm programs and all specified
[output formats].
[output formats]: http://www.graphviz.org/doc/info/output.html
* Functions to convert [FGL] graphs to and from the internal Dot
representations.
[FGL]: http://web.engr.oregonstate.edu/~erwig/fgl/haskell/
* The ability to augment Dot and FGL graphs with positioning
information by round-trip passing through Graphviz.
### Is the API of _graphviz_ stable? ###
For the most part, yes: the only items that are likely to change in
the future are those with bugs/errors or if a radically better way of
doing things is found. For most uses, however, the API should not
change for the foreseeable future.
Note that _graphviz_'s version numbers follow the
[package versioning policy]; this means that you can immediately tell
when the API has had a backwards-incompatible change by comparing the
first two elements of the version. However, these changes won't
affect most users.
[package versioning policy]: http://www.haskell.org/haskellwiki/Package_versioning_policy
### What aspects of Dot syntax and attributes are covered? ###
It's easier to state which aspects of Dot [syntax] and [attributes]
_aren't_ covered:
#### Overall syntax items not covered ####
* Cannot specify a sub-graph as an end point in an edge;
* Comments, pre-processor lines and split lines are (currently) not
supported within HTML-like labels.
* _graphviz_ is currently locale-specific: Dot graphs are meant to be
encoded in UTF-8 by default unless specified to be Latin-1, but this
isn't verified or checked. Dot code that is parsed in is assumed to
be in UTF-8; in future this will be enforced (both for printing and
parsing purposes).
* Graphviz is more liberal in accepting "invalid" values
(e.g. accepting a floating-point value when only integer values are
meant to be accepted); _graphviz_ is more strict in this aspect (and
will indeed throw an exception if it cannot parse something
properly).
* No extensions (e.g. postscript-specific attributes) are available.
#### Attribute and value items not covered ####
* The global `orientation` attribute is not defined; however its
behaviour is duplicated by the `rotate` attribute.
* The deprecated `overlap` algorithms have not been defined.
* `pointf` and `point` values have been combined into one datatype;
however the optional `!` and third value for `point` values is not
accepted.
* Only polygon-based `shape`s are available.
* The default `layersep` is used when printing and parsing
`layerRange` and `layerList` values; this will be fixed in a future
release (when state-based printing and parsing is implemented).
* The `/ssss/yyyy` and `//yyyy` forms of printing and parsing `color`s
are not yet available.
#### Available items of note ####
There are a few items of note that are available that are worthy of
special note (as they may not be immediately obvious from the
generated documentation):
* _graphviz_ is able to parse (but not print) the following special
aspects of specifying edges in Dot code:
- The `node:port` method of specifying of head/tail `portPos`
values.
- Stating multiple edges with common interior nodes (e.g. `a -> b
-> c`).
- Stating edges with a grouping of nodes (e.g. `a -> {b c}`).
* Sub-graphs are specified as being clusters when the subgraph name
starts with either `"cluster"` or `"cluster_"`; note that this
prefix is removed when determining the subraph's name for the
internal datatypes.
* Anonymous subgraphs (where not even the `subgraph` keyword is
specified) are also parseable.
* HTML-like and record labels are available, and feature proper
escaping/unescaping when printing/parsing.
Getting _graphviz_ and more documentation
-----------------------------------------
### Where can I obtain _graphviz_? ###
The best place to get _graphviz_ is from its [HackageDB] page.
[HackageDB]: http://hackage.haskell.org/package/graphviz
### Where can I find the API documentation for _graphviz_? ###
Also on its [HackageDB] page.
### Is it safe to install and use _graphviz_ from its darcs repository? ###
No; unlike other projects I make no guarantees as to the stability of
the live version of _graphviz_. Whilst the [darcs] [repository] is
_usually_ stable, it's often in a state of flux and at times patches
that break the repository are recorded (when it's simpler/cleaner to
break one patch into several smaller patches).
[darcs]: http://darcs.net/
[repository]: http://code.haskell.org/graphviz/
### How is _graphviz_ licensed? ###
_graphviz_ is licensed under a [3-Clause BSD License] (note that the
ColorBrewer Color Schemes found in `Data.GraphViz.Attributes.Colors`
are covered under
[their own license](http://graphviz.org/doc/info/colors.html#brewer_license)).
[3-Clause BSD License]: http://www.opensource.org/licenses/bsd-license.php
Simplistically, this means that you can do whatever you want with
_graphviz_ as long as you cite both myself and [Matthew Sackman] (the
original author) as being the authors of _graphviz_. However, I would
appreciate at least an [email] letting me know how _graphviz_ is being
used.
[Matthew Sackman]: http://www.wellquite.org/
[email]: mailto:Ivan.Miljenovic+graphviz@gmail.com
### Where can I find more information on _graphviz_? ###
From its [home page].
[home page]: http://projects.haskell.org/graphviz/
### Are there any tutorials on how to use _graphviz_? ###
There will be soon.
### What other packages use _graphviz_? ###
This is a list of all known packages that use _graphviz_: if you know
of any others please let me know and I'll add it to the list.
* [Graphalyze](http://hackage.haskell.org/package/Graphalyze)
* [SourceGraph](http://hackage.haskell.org/package/SourceGraph)
### What is the history of _graphviz_? ###
_graphviz_ was originally written by [Matthew Sackman] (if you want
his reasons for doing so, you'll have to ask him yourself) with the
first known release being on 10 July, 2008. In 2008 I (Ivan
Miljenovic) needed a library that provided bindings to Graphviz with
clustering support; at the time _graphviz_ was the most fully featured
and closest to what I wanted, so I submitted a patch that provided
support for both clustering and undirected graphs.
In April 2009, Matthew wanted to step down from maintaining _graphviz_
and asked if I wanted to take over. Since then the library has been
almost completely re-written with greatly improved coverage of the Dot
language and extra features. However, the original outline of the
library still remains.
Using _graphviz_
----------------
### Can I start using _graphviz_ without knowing anything about Graphviz? ###
Unfortunately, no: the layout and design of _graphviz_ is heavily
based upon the Dot language and the various [attributes] that Graphviz
supports. As such, you can't just suffice on the documentation
available in _graphviz_ (unless you're doing something _very_
simplistic).
### Can I just use _graphviz_ without reading its documentation? ###
You should _at least_ read the various messages about possible
ambiguities, etc. at the top of each module and for the attributes you
use before you use _graphviz_.
### Do I need to have Graphviz installed to use _graphviz_? ###
Technically, no if you're only dealing with the Dot language aspects.
However, usage of the functions in the Commands module, or the
augmentation of pretty-printing functions in the GraphViz module _do_
require Graphviz to be installed.
### Why didn't you use FFI to bind to the Graphviz library? ###
Because I just kept working where [Matthew Sackman] left off and it
was already using Graphviz's tools rather than the actual library.
However, most other language bindings (for Python, Perl, etc.) seem to
do the same: generate Dot code and pass that to the relevant tool.
This, however, does provide a fortunate side effect where the ability
to print and parse Dot code means that _graphviz_ can be used for more
than just visualising graphs created solely in Haskell: it can also
import pre-defined graphs, or else generate Dot code for use with
other tools.
### What's the difference between DotGraph and GDotGraph? ###
The layout of `DotGraph` matches the output of `dot -Tcanon`. It has
a fixed layout which makes it easier to reason about and get
sub-components.
`GDotGraph` on the other hand is more liberal in its layout, allowing
you to put statements in any order you please. This is useful in
cases where you want to use the common Graphviz "hack" of specifying
global attributes that don't apply to sub-graphs _after_ the
sub-graphs in question.
### What's the best way to parse Dot code? ###
In both cases below, you should use the `parseDotGraph` function to
parse the Dot code: this is because it will strip out comments and
pre-processor lines and join together split lines (if any of these
remain the parser will fail). Also, if you are not sure what the type
of the nodes are, use either String or else the `GraphID` type as it
explicitly caters for both Strings and numbers (whereas just assuming
it being a String will result in numbers being stored internally as a
String).
If you can, first run `dot -Tcanon` on the Dot code and parse it as a
`DotGraph` value. This is because `DotGraph` types are easier to deal
with.
If, however, this isn't possible (e.g. it uses an image that isn't in
the current working directory) then use the `GDotGraph` type.
### There are too many attributes!!! Which ones should I use? ###
The following attributes are easy to use and recommended:
* `ArrowHead` and `ArrowTail` (for directed graphs) to set the styles
of the ends of edges: note that in Graphviz parlance, "Head" refers
to the end node and "Tail" refers to the start node of the edge (see
below).
* When wanting to use different colours, use the following criteria to
pick the correct attribute. Note: for the first two, you should
also have `SItem Filled []` set as one of the `Style` values for
that item.
- `BgColor` to set the background colour of a graph/cluster.
- `FillColor` to set the background colour for a node.
- `Color` to set the colour of an edge; if you supply more than
one value then the edge is drawn using parallel splines/lines
(one per colour in the list).
- `PenColor` to set the colour of the bounding box for a cluster.
When choosing a `Color` value for one of the above, it is better to
use one of the `X11Color` values (note: these are **not** the same
as the standard X11 colours) or - if you have to - one of the manual
colours over a `BrewerColor`, as they come under a different license
and have no real standard on what the values are.
* `Label`: `StrLabel` can be used for both nodes and edges; the other
two only for nodes. `RecordLabel` and `HtmlLabel` provide ways of
having more fine-grained control over a node's layout with different
sub-components, etc.; in most cases these won't be needed.
* `Rank`: this lets you control relative placement of sub-graphs and
clusters.
* `Shape`: Use `Record` and `MRecord` for `RecordLabel` labels; feel
free to use any other ones at any time (though you probably want to
use `PlainText` for `HtmlLabel` labels).
* `Style`: use this to set line types, etc. for nodes and edges. You
should **not** use a `DD` (device-dependent) value.
The following attributes are **not** recommended for use:
* `Charset`: the only accepted options are `"UTF-8"` and `"Latin-1"`,
but in future _graphviz_ will not contain this attribute and will
only allow UTF-8 usage.
* `Color` for anything except edge colours.
* `ColorScheme`: just stick with X11 colours.
* `Comment`: pretty useless, but will interfere with the augmentation
functions (since they use the `Comment` attribute to distinguish
between multiple edges).
### Can I use any attribute wherever I want? ###
No: attributes are all defined in one big datatype for the sake of
simplicity, but not all attributes are valid in all places. Read the
documentation (either in Graphviz or _graphviz_) to determine which is
suitable where.
### How can I use _graphviz_ to visualise non-FGL graphs? ###
At the moment, you unfortunately have to write your own manual
conversion functions (see `graphToDot`, etc. in the GraphViz module
for ideas on how to do this). In future, it should be possible to
convert any graph-like datatype into a `DotGraph` (this requires me to
go write another library first...).
### How can I use/process multiple graphs like Graphviz does? ###
At one stage, _graphviz_ supported dealing with lists of `DotGraph`s;
however, it was found to be faster to deal with each graph
individually rather than try to get Graphviz to deal with them all in
one go. In future, once the problem causing this has been tracked
down and fixed this feature will be returned.
### How can I use custom datatypes for node IDs? ###
The important thing here is to ensure that your custom datatype has
defined instances of `PrintDot` and `ParseDot`. Probably the easiest
way of doing this is to have functions that convert between your type
and `String` and let graphviz determine how to print and parse those.
Here is an example of a more difficult type that should be printed
like "1: Foo":
~~~~~~~~~~~~~~~~~~~~ {.haskell}
data MyType = MyType String Int
instance PrintDot MyType where
unqtDot (MyType s i) = unqtDot i <> colon unqtDot s
-- We have a space in there, so we need quotes.
toDot = doubleQuotes . unqtDot
instance ParseDot MyType where
parseUnqt = do i to:b` where
the nodes "from" and "to" are defined with either `RecordLabel` or
`HtmlLabel` labels and have different sections; the edge is then drawn
from the "a" section of the "from" node to the "b" section of the "to"
node.
Whilst _graphviz_ can parse this, you can't define this yourself:
instead, do it the manual way:
~~~~~~~~~~~~~~~~~~~~ {.haskell}
DotEdge "from" "to" True [ TailPort (LabelledPort (PN "a") Nothing)
, HeadPort (LabelledPort (PN "b") Nothing)
]
~~~~~~~~~~~~~~~~~~~~
Note where `TailPort` and `HeadPort` are used; the next question
explains this.
### Is there anything else I should know? ###
A few other things of note that you should know about:
* For an edge `a -> b`, Graphviz terms "a" to be the _tail_ node and
"b" to be the _head_ node.
* When creating `GraphID` values for the graphs and sub-graphs, you
should ensure that they won't clash with any of the `nodeID` values
when printed to avoid possible problems.
* It is a good idea to have unique IDs for sub-graphs to ensure that
global attributes are applied only to items in that sub-graph and so
that clusters aren't combined (it took me a _long_ time to find out
that this was the case).
* You should specify an ID for the overall graph when outputting to a
format such as SVG as it becomes the title of that image.
* It is possible to specify a graph as being directed/undirected but
having individual edges being the opposite; care should be taken to
avoid this (this possible issue may be resolved in future).
* Graphviz allows a node to be "defined" twice with different
attributes; in practice they are combined into one node. Running
Dot code through `dot -Tcanon` before parsing removes this problem.
* Several attributes are defined with taking a list of items; all of
these assume that the provided lists are non-empty (sub-values are a
different story).
* If a particular Dot graph is not parseable, the parser throws an
error rather than failing gracefully.
Design Decisions
----------------
### Why does _graphviz_ use Polyparse rather than Parsec? ###
Short answer: because _graphviz_ was already using [Polyparse] when I
started working on it (and I hadn't done any parsing before so I had
no preference either way).
[Polyparse]: http://www.cs.york.ac.uk/fp/polyparse/
Longer answer: Polyparse has several advantages I feel over [Parsec]:
* Simpler types.
* Avoids the whole "but Parsec-3 is slower than Parsec-2" debate (with
its associated baggage/problems).
* Few inbuilt combinators: since there is no inbuilt `character`
parsing combinator, there are no problems with _graphviz_ using its
own case-less one.
* [Easier backtracking](http://www.cs.york.ac.uk/fp/polyparse/#how)
[Parsec]: http://hackage.haskell.org/package/parsec
### Why do you have two different representations of Dot graphs? ###
_graphviz_ has
[two different representations](#whats-the-difference-between-dotgraph-and-gdotgraph)
of Dot graphs. Apart from the reasons given before, `DotGraph` was
the original representation, whereas `GDotGraph` was only introduced
in the 2999.8.0.0 release.
Note, however, that I was thinking of adding something like
`GDotGraph` back around the time of the
[2999.0.0.0 release](http://www.haskell.org/pipermail/haskell-cafe/2009-July/064436.html),
yet
[people didn't like the idea](http://www.haskell.org/pipermail/haskell-cafe/2009-July/064442.html).
As such, `GDotGraph` is there if anyone needs/wants to use it, but
usage of `DotGraph` is recommended/preferred.
### Why are only FGL graphs supported? ###
Love them or hate them, [FGL] currently provides the best graph
datatype and library available for Haskell at this time. As such, if
any one graph type had to be chosen to have conversion functions
written for it then FGL is the best option. Furthermore, I needed FGL
graph support (which is the much more important reason!).
### Why are the version numbers so high? ###
To make sure the latest release has the highest version number:
Matthew Sackman originally made releases with date-based versioning,
but when I switched to using the [package versioning policy] I had to
change this. I could have started with 2010.x.y.z or so, but at the
time I had initial hopes of introducing compatibility with other
graphs (not just [FGL] ones) soon and wanted to make that the
3000.0.0.0 release; however that has not yet come to pass.
### Why do you use the American spelling of colour in _graphviz_? ###
Because that's how Graphviz spells it, and I was following upstream to
avoid confusion.
Bugs, Feature Requests and Development
--------------------------------------
### Do you have any future plans for _graphviz_? ###
Yes, I do! See the TODO file for more information.
### Does _graphviz_ have a test suite? ###
Yes, there is: to get it, you have to build it with the `test` flag
enabled; for example:
~~~~~~~~~~~~~~~~~~~~ {.bash}
cabal install graphviz --flags=test
~~~~~~~~~~~~~~~~~~~~
Then run the `graphviz-testsuite` executable. This test suite uses
[QuickCheck] to ensure that _graphviz_ can parse the Dot code it
generates (as well as a few other things). Note that it isn't
perfect: there are no guarantees that the Dot graphs that are
generated are indeed valid, and those more extensive tests are not yet
available.
[QuickCheck]: http://hackage.haskell.org/package/QuickCheck
Furthermore, you can do more controlled testing to try and track down
the source of a bug as the above flag will also expose several
testing modules which give you access to the various tests used as
well as the `Arbitrary` instances for use with [QuickCheck].
For proper testing of real-life Dot code, there is also the
`TestParsing.hs` script that comes in the _graphviz_ tarball (but is
not installed). Once you have _graphviz_ installed you can just run
this script, passing it any files containing Dot graphs you wish to
test. It will attempt to parse each Dot graph as a `GDotGraph`, and
then test to see if the canonicalised form is parseable as a
`DotGraph`.
### I've found a bug! ###
Oh-oh... please [email] me the specifics of what you were doing
(including the Dot file in question if it's a parsing problem) and
I'll get right on it.
### I have a feature request. ###
Is it in the TODO? If not, [email] me and I'll consider implementing
it (depending on time and how well I think it will fit in the overall
library).
### I want to help out with developing _graphviz_. ###
Great! Whether you have a specific feature in mind or want to help
clear the TODO list, please [email] me to check with what you're doing
(who knows, I could already be implementing that very feature).
Once we've discussed what you're going to do, first get yourself a
copy of the darcs repository:
~~~~~~~~~~~~~~~~~~~~ {.bash}
darcs get --lazy http://code.haskell.org/
~~~~~~~~~~~~~~~~~~~~
Once you've made your changes, make sure you build and run the
testsuite (and ensure it passes!). Then record the patch[es] and
`darcs send` them. I'll then review them and if I'm happy with them,
I'll apply them.
### What is the purpose of the AttributeGenerator.hs file? ###
Graphviz has a large number of attributes. Rather than try to edit
everything manually each time I want to change how I use the large
`Attribute` datatype, the AttributeGenerator script generates the
datatype, instances, etc. for me.